The present project investigated thermoregulatory functions of humans subjected to heat exposure for several hours limited to a fixed time daily.The food ingestion and sleep-awake cycle of volunteers were controlled throughout the experiments. For heat accliamtion, the subjects were exposed to an ambient temperature (Ta) of 46ﾟC for 4 hrs (14 : 00-18 : 00 h) daliy.Experiment 1 : Core temperature (Tcor) of the subjects were measured for 24 h at a constant Ta of 27ﾟC with or without heat acclimation. The pattern of day-night variations of Tcor was altered by heat acclimation, i.e., the Tcor levels were maintained at low levels in the afternoon.Experiment 2 : The subjects were seated in a chair at Ta of 28ﾟC.Both legs were immersed in a warm water and sweating was induced. The procedure was repeated twice in the day, once in the morning and once in the afternoon, before and after heat acclimation. The latency for thermal sweating was shortented and the threshold Tcor for sweating was lowered by heat acclimation only in the afternoon.The results give evidence that in humans, repeated heat exposure limited to a fixed time daily lowers Tcor and alters thermoregulatory functions during the period when the subjects were previously exposed to heat.